For the last 20-30 years, temperature sensitive paints have proved useful in the development of high temperature turbine and combustion hardware. Thermal paints permanently change color when the test component on which they have been applied exceeds a predetermined temperature governed by the chemical composition of the paints themselves. By observation and analysis of the paint coloration pattern after a test run has been completed, it is possible to obtain a comprehensive map of the isotherms along the painted test pieces. Such paint is available for example from Thermindex Chemicals & Coatings LTD.
The painted components are assembled in a rig/engine prior to any testing and handled using clean nylon hand gloves to prevent contamination of the painted surfaces.
If the test part is part of a gas turbine engine, there should be no compressor wash or wet motor before the test run so that the painted surfaces remain uncontaminated. Whenever possible, engine light-up should be made using natural gas fuel. The fuel flow should be kept to a minimum during ignition to avoid turbine torching, and light-up should preferably occur on the first start. When a test run encompasses turbine nozzle and blade paint tests, the running time at full-load conditions should be achieved as quickly as possible and time spent at full load should not exceed 5 minutes to avoid paint removal by the scrubbing action of the exhaust gases. When paint testing heavy structures such as disks, diaphragms or casings, the running time should be 5 hours.
Combustor liners can undergo much longer exposure times without adverse effects. Color change calibrations for thermal paint are available up to some 50 hours exposure time although 30 minutes is the test norm. Estimates of temperature contours after a series of short "runs" even at nominally the same conditions can be misleading.
At the conclusion of a test run, the engine fuel should be cut-off at the maximum load conditions in order to reduce any effect of transient temperature exposure. The engine should then be motored at cranking speed for 15 minutes to cool the hot section parts. This procedure ensures that the heat lost by cooling at all locations is greater than the heat gained by transfer from the hotter to the cooler parts of any test section. If this precaution is disregarded, it may lead to a certain amount of heat-soaking and parts attaining a higher temperature during the cooling phase than they had reached during the test itself.
Thermal indicating paints change color in response to both time and temperature. In effect, this means when thermal paints are subjected at temperature to a time duration, the longer the time, the lower the temperature required to change the paint to the next color.
The painted objects should be brought to their operating temperatures as quickly as possible and conditions maintained constant for a period of time. The test time period is very important as it can be seen from the calibration charts above that the color change points vary with heating time.
Calibrated test strips should be taken from time to time to confirm the continuing calibration of the batch of paint acquired. Where possible, the test strips should be the same material and thickness as the engine component. Exposure time must be the same and allowances in paint calibrations should be made for differences in flow vitiation and the time taken for the test strip or tally to reach the working temperature in the laboratory.
FIG. 1 illustrates a fan blade from a gas turbine engine that has been painted with temperature sensitive paint and run in the gas turbine engine for a predetermined length of time. The illustrated areas indicate the different operating temperatures of the various areas of the fan blade in operation. As will be understood by those skilled in the art, accurate interpretation of the thermal paint pattern is an obvious prerequisite to the success of the manual measuring technique. Manual interpretation is subjective and requires an experienced operator who physically marks with a pencil the various color zones of a paint tested component with color boundary lines and then assigns quantitative values to each isothermal area. A single color in a series can span a temperature range of over 150.degree. F., so it may be necessary to interpolate temperatures within a single color change.
The present invention is directed to overcoming one or more of the problems set forth above.